Microphones (mics) are provided in a wide variety of electronic applications. As the need to reduce the size of many components continues, the demand for reduced-size mics continues to increase as well. This has lead to comparatively small mics, which may be micromachined according to technologies such as micro-electromechanical systems (MEMS) technology.
One type of mic is a micromachined piezoelectric mic. The piezoelectric mic includes a layer of piezoelectric material between two conductive plates (electrodes). An acoustic wave incident on the membrane of the mic results in the application of a time varying force to the piezoelectric material. Application of this force to piezoelectric material results in induced stresses in the piezoelectric material, which in-turn creates a time-varying voltage signal across the material. This time-varying voltage signal may be measured by sensor circuits to determine the characteristics of the incident acoustic wave. Alternatively, this time-varying voltage signal may produce a time-varying charge that is provided to sensor circuits that process the signal and determine the characteristics of the incident acoustic wave.
The capacitance of a piezoelectric mic may be represented:
                              C          =                                    A                              κ                ⁢                                                                  ⁢                                  ɛ                  0                                                      d                          ,                            (                  Eqn          .                                          ⁢          1                )            
where A is the common area of the (plate) electrodes of the mic, κ is the dielectric constant of the piezoelectric material, εo is the electrical permittivity of free space and d is the separation distance between the plate electrodes of the mic.
As is known, charge across a capacitor may be representedQ=C·V  (Eqn. 2)
In a piezoelectric mic, capacitance is substantially fixed, and the voltage varies, thereby resulting in a change in charge according to the relation:ΔQ=CΔV  (Eqn. 3)
As is known, there is a need to provide suitable sensitivity with the mic. This proves an ever-increasing challenge with smaller mics. From Eqn. 1, it can be appreciated that by reducing the distance (d) and increasing the dielectric constant (κ) of the piezoelectric material the capacitance of a piezoelectric mic can be comparatively large. The higher level of capacitance of a piezoelectric mic may simplify sensor signal processing circuit design.
While piezoelectric mics are useful in certain applications, there are drawbacks to known piezoelectric mics. For example, assuming that the c-axis of the piezoelectric material does not significantly change across the membrane of the mic, the voltage sensitivity (V/q) couples through the lateral stress, σy, and is ideally proportional thereto. Moreover, in an ideal clamped, thin plate piezoelectric mic under uniform load, there are at least two regions of differing curvature.
In a first region, the top of the piezoelectric layer may be in compressive stress (negative lateral stress), and the bottom of the piezoelectric layer may be in tensile stress (positive lateral stress). Consequently, this first region has a first voltage polarity. In a second region, the top of the piezoelectric layer may be in tensile stress, and the bottom of the piezoelectric layer may be in compressive stress. Consequently, this second region has a second voltage polarity that is opposite the polarity of the first region. If the mic has an upper electrode continuous across both the first and second regions and a lower electrode continuous across the first and second regions, then the opposite polarities of the first and second region may result in a normalization of charge (and hence electrical potential difference) across the first and second regions. This charge normalization can result in a lower sensitivity. As will be appreciated, this reduction in sensitivity is undesirable, especially in comparatively small-dimension mics.
There is a need, therefore, for a transducer structure and an electronic device that address at least the shortcomings described above.